Traditional IC sockets are generally constructed of an injection molded plastic insulator housing which has stamped and formed copper alloy contact members stitched or inserted into designated positions within the housing. These contact members can be in a flat or “blank” format, or they can be produced with a series of forms, bends, and features to accommodate a desired function such as retention within the plastic housing.
The designated positions in the insulator housing are typically shaped to accept and retain the contact members. The assembled socket body is then generally processed through a reflow oven which melts and attaches solder balls to the base of the contact member. During final assembly onto a printed circuit board (“PCB”), the desired interconnect positions on the circuit board are printed with solder paste or flux and the socket assembly is placed such that the solder balls on the socket contacts land onto the target pads on the PCB. The assembly is then reheated to reflow the solder balls on the socket assembly. When the solder cools it essentially welds the socket contacts to the PCB, creating the electrical path for signal and power interaction with the system.
During use, this assembled socket receives one or more packaged integrated circuits and connects each terminal on the package to the corresponding terminal on the PCB. The terminals on the package are held against the contact members by applying a load to the package, which is expected to maintain intimate contact and reliable circuit connection throughout the life of the system. No permanent connection is required. Consequently, the packaged integrated circuit can be removed or replaced without the need for reflowing solder connections.
As processors and electrical systems evolve, several factors have impacted the design of traditional sockets. Increased terminal count, reductions in the terminal pitch (i.e., the distance between the contacts), and signal integrity have been main drivers that impact socket and contact design. As terminal count increases, the IC packages get larger due to the additional space needed for the terminals. As the IC packages grow larger the relative flatness of the IC package and corresponding PCB becomes more important. A certain degree of compliance is required between the contacts and the terminal pads to accommodate the topography differences and maintain reliable connections.
IC package manufacturers tend to drive the terminal pitch smaller so they can reduce the size of the IC package and reduce the flatness effects. As the terminal pitch reduces, however, the surface area available to place a contact is also reduced, which limits the space available to locate resilient contact members that can deflect without shorting to an adjacent contact member.
For mechanical reasons, longer contact members are preferred because they have desirable spring properties. Long contact members, however, tend to reduce the electrical performance of the connection by creating a parasitic effect that impacts the signal as it travels through the contact. Long contact members also require thinner walls in the housing in order to meet pitch requirements, increasing the risk of housing warpage and cross-talk between adjacent contact members. The demands of pitch reduction often reduce the available area for spring features. Often such contact members require retention features that add electrical parasitic effects.
The contact members are typically made from a selection of Copper based alloys. Since copper oxidizes, the contacts are typically plated with nickel to prevent migration, and a final coating of either a precious metal like gold or a solder-able metal such as tin. In very cost sensitive applications, the contacts are sometimes selectively plated at the interface points where they will connect to save the cost of the plating.
The copper based alloys also represent a compromise of material properties. For example, the spring constant of copper alloys is less than stainless steel, and the conductivity of copper alloys is less than pure copper or silver. Copper also oxidizes readily, so plating must be applied to at least a portion of the contact to improve the corrosion resistance.
One alternative to traditional resilient contact members are composite contacts containing tiny particles of silver molded into a silicone matrix. When compressed, the silver particles touch each other can create electrical contact. These composite contact members suffer from high contact resistance due to the silicone material interfering with the conductive path.
Next generation systems will operate above 5 GHz and beyond. Traditional sockets and interconnects will reach mechanical and electrical limitations that mandate alternate approaches.